The present application relates to a technique for imaging fluid flow noninvasively using MRI with radio frequency arterial spin labeling and, more particularly, to a labeling technique that permits extension of this technique to a multislice or volume examination.
Magnetic Resonance Imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on principles of nuclear magnetic resonance (NMR), a spectroscopic technique used to obtain microscopic chemical and physical information about molecules.
MR (magnetic resonance) techniques may be used to study fluid flow, such as, for example, blood flow and blood perfusion in tissue. One of many possible applications is to study blood perfusion in the human brain.
Typically, during MR studies, images are taken of a volume of interest or of xe2x80x9cslicesxe2x80x9d in a region of interest. A slice may be defined as a relatively thin region, which may be imaged in a single image. Each slice is said to be composed of several volume elements called voxels. Traditionally, volume of a voxel may be approximately 3 mm{circumflex over ( )}3, however, it may vary depending on the application. The magnetic resonance image is said to be composed of several picture elements called pixels. The intensity of a pixel is proportional to the NMR signal intensity of the contents of the corresponding volume element, or voxel, of the object being measured. Present-day MR studies typically call for more than one slice image to be taken, or one or more images of the volume of the region of interest to be taken.
Magnetic resonance imaging is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. The human body, for example, is composed in large part of water, which has many hydrogen atoms. Hydrogen nuclei have an NMR signal in the presence of the magnetic field, and after being perturbed by RF (radiofrequency) irradiation. Protons in the hydrogen atoms (and in other atoms) possess a property called spin, which can be thought of as a small magnetic field and which will cause the nucleus to produce an NMR signal. Water consists in large part of hydrogen atoms. There are known apparatuses for detecting water in tissue using the NMR signal of hydrogen nuclei. Using these basic principles, fluid content may be measured in a variety of substances or tissues.
In order to detect fluid flow or perfusion in a particular region of interest, fluid flowing into that region may be xe2x80x9clabeledxe2x80x9d by reversing, or perturbing, the spins of the protons of the fluid in some region that is xe2x80x9cupstreamxe2x80x9d from the region of interest, and then detecting the labeled fluid when it flows through or is perfused in the region of interest.
Such studies have previously been performed by applying a constant magnetic field to the fluid and applying RF irradiation to label the spins. In order to spatially isolate labeling areas, a magnetic field gradient may be used. Spatially isolating a particular area means applying, in the particular area, a magnetic field different in strength from the magnetic field in other areas. By using the magnetic field gradient to spatially isolate particular areas, only fluid in a predetermined region may be labeled. However, magnetization transfer effects and other unrelated errors may interfere with the procedure by causing more than just the xe2x80x9cupstreamxe2x80x9d fluid to be labeled. In order to account for such effects, a control procedure may be used, where a magnetic field gradient and RF irradiation are applied so as not to label the spins, but to mimic the unrelated effects.
One of the concerns in performing such a study is possible power deposition resulting from application of RF irradiation. Typically, power deposition is higher where the present magnetic field and the intensity of RF irradiation are higher. There are boundaries on the power that may be deposited in the human body without harming the body. However, stronger RF irradiation results in higher sensitivity that in turn produces better results. Therefore, there is a need for a higher-sensitivity method for performing fluid flow studies, especially multi-slice or volume studies, in the presence of strong magnetic fields, while taking into account maximum power deposition considerations.
The present invention relates to a method and apparatus for conducting a magnetic resonance fluid flow study by conducting a labeling procedure and a control procedure and combining datasets from those procedures to create a dataset for the fluid flow study. An amplitude modulated magnetic field gradient and amplitude modulated RF irradiation may be applied during a labeling procedure. Likewise, amplitude modulated RF irradiation may be applied during control procedure. An amplitude modulated magnetic field gradient may also be applied during the control procedure.
An envelope for modulating the label RF irradiation may be an absolute value of an envelope for modulating the control RF irradiation, such that an average amplitude for the label RF irradiation is not zero. The average amplitude for the label RF irradiation may be positive or negative, depending on the requirements of a particular application. An average amplitude for control RF irradiation may be zero or near zero.
An amplitude modulated magnetic field gradient may also be applied during the control procedure. An envelope for modulating the control magnetic field gradient may be similar to the envelope for modulating control RF irradiation.
The control RF irradiation may further be frequency modulated. An envelope for frequency modulation of the control RF irradiation may be similar to the envelope for amplitude modulation of the control RF irradiation. In one embodiment of the invention, the envelope for amplitude modulation of the control RF irradiation may be a modified square wave. The envelope for the label RF irradiation may be the absolute value of the modified square wave.
An image of fluid flow may be generated by subtracting the label dataset from the control dataset. In an alternative embodiment of the invention, the image of the fluid flow may further be modified by subtracting artifacts of systematic errors from the image.
Another aspect of the invention is a method for compensating for magnetization transfer effects by applying an amplitude modulated magnetic field gradient and amplitude modulated RF irradiation during a label procedure and applying an amplitude modulated control magnetic field gradient and amplitude modulated control RF irradiation during a control procedure. Such compensation for the magnetization transfer effects may be performed in a context of an MR study, such as, for example, an MR imaging study. Furthermore, RF irradiation may be frequency modulated during the label and/or control experiments. An envelope for label RF irradiation for frequency and amplitude modulation may be similar and may be similar to an envelope for amplitude modulation for label magnetic field gradient. An envelope for control RF irradiation for frequency and amplitude modulation may be similar and may be similar to an envelope for amplitude modulation of the control magnetic field gradient. The envelope for label RF irradiation modulation may be an absolute value of the envelope for the control RF irradiation. The envelope for the control RF irradiation may be a modified square wave. In an alternative embodiment, the envelope for the control RF irradiation may be a function other than the square wave, as long as an average control RF irradiation amplitude is at or near zero.
In yet another aspect of the invention, a magnetic resonance apparatus may be constructed and arranged to perform fluid flow imaging. Such apparatus may apply amplitude modulated label RF irradiation and a magnetic field gradient during the labeling procedure. Furthermore, such apparatus may apply amplitude modulated label RF irradiation and a magnetic field gradient during the control procedure. The envelope for applied control magnetic field gradient may be similar or identical to the envelope for applied control RF irradiation. The envelope for applied label RF irradiation and the magnetic field gradient may be the absolute value of the envelope for the control RF irradiation.
In yet another embodiment of the invention, a computer readable medium may be provided. Encoded on the computer readable medium may be instructions that, when executed, direct a method for generating an image of fluid flow obtained from a magnetic resonance system. Such a method may comprise combining a control image received from the control procedure where the amplitude modulated RF irradiation and the magnetic field gradient were applied with a label image received from the labeling procedure where the amplitude modulated RF irradiation and the magnetic field gradient were applied. The control and label images may be combined to create the image of fluid flow by subtracting the label image from the control image.
In another embodiment of the invention, the image of the fluid flow may be further modified by subtracting artifacts of residual or system errors. A dataset for the residual or system errors may be obtained by performing labeling and control procedures in a region different from the region being imaged such as upstream or downstream from the region being imaged. The fluid flow image may be further modified as appropriate for a particular application.